A technique for use in producing various semiconductor devices is known in which a conductive film for use as an electrode, wiring, or the like is formed using a conductive paste. Formation of such a conductive film will be described with reference to a solar cell element as a typical example of a semiconductor device.
A common solar cell element includes a semiconductor substrate, a diffusion layer, an antireflective layer, a back electrode, and a front electrode. The semiconductor substrate is doped with a trace amount of semiconductor impurity. For example, when silicon is used as a semiconductor material, a p-type semiconductor is obtained by doping silicon with a trivalent element such as boron, and an n-type semiconductor is obtained by doping silicon with a pentavalent element such as phosphorus. The diffusion layer is formed on the light-receiving surface (front surface) side of the semiconductor substrate.
The diffusion layer is a region having a conductivity type opposite to that of the semiconductor substrate. For example, when the semiconductor substrate is of p-type, an n-type diffusion layer is formed by diffusing a pentavalent element into the light-receiving surface. On the other hand, when the semiconductor substrate is of n-type, a p-type diffusion layer is formed by diffusing a trivalent element into the light-receiving surface. The antireflective layer is formed on the light-receiving surface side of the diffusion layer to prevent the reflection of incident light and protect the front surface of the solar cell element.
The front electrode is formed on the surface of the diffusion layer, and the back electrode is formed on the back surface of the semiconductor substrate. It is to be noted that a BSF (Back Surface Field) layer may be formed on the back surface of the semiconductor substrate so that the back electrode is formed on the BSF layer. These front electrode and back electrode are formed as conductive films using a conductive paste. Specifically, a conductive paste is printed and dried to form a pattern of the front electrode or back electrode, and is then fired under predetermined conditions to form an electrode made of a conductive film having a predetermined pattern. The front electrode and the back electrode may be formed using the same conductive paste to have the same composition or may be formed using different conductive pastes to have their respective suitable compositions.
In the solar cell element, as described above, the antireflective layer is formed on the surface of the diffusion layer. Therefore, when the front electrode is formed, the conductive paste needs to achieve good fire-through upon firing. Fire-through is a phenomenon in which glass frit contained in a conductive paste acts on an antireflective layer upon firing to dissolve and remove the antireflective layer so that a front electrode and a diffusion layer come into direct contact with each other and an ohmic contact is obtained.
If a stable ohmic contact cannot be obtained between the front electrode and the diffusion layer, a solar cell tends to have a reduced fill factor (FF) due to an increase in series resistance. The conversion efficiency of a solar cell is a value determined by multiplying open-circuit voltage by short-circuit current density by FF. Therefore, when FF is reduced, the conversion efficiency is reduced.
In order to achieve good fire-through upon firing, techniques have been heretofore proposed in which various additives are added to a conductive paste. For example, Patent Document 1: WO 2007/125879 A discloses a conductive paste for solar cell electrodes which contains, as an additive in addition to an organic binder, a solvent, conductive particles, and glass frit, (1) a combination of a substance that changes to gas at a temperature in the range of 150 to 800° C. and a metal oxide, (2) a combination of an organic metal compound and a metal oxide, or (3) a compound containing Al, Ga, In, or Tl.
Further, Patent Document 2: WO 2009/134646 A discloses a thick-film conductive composition (conductive paste) containing, in addition to a conductive powder, glass frit, and an organic medium, (1) a first additive selected from the group consisting of (i) a bismuth-containing additive and a phosphorus-containing additive, (ii) a metal oxide of one or more of bismuth and phosphorus, (iii) any compound that can generate the metal oxides of (ii) upon firing, and (iv) mixtures thereof and (2) a second additive selected from the group consisting of (i) a metal oxide additive and (ii) a compound that can generate a metal oxide upon firing.
Further, the present inventors have proposed a conductive paste for forming an electrode disclosed in Patent Document 3: WO 2010/016186 A, which contains, in addition to conductive particles, an organic binder, a solvent, and glass frit, (1) an organic compound containing an alkaline-earth metal, (2) a low-melting-point metal, (3) a low-melting-point metal-based compound, (4) a combination of an organic compound containing an alkaline-earth metal and a low-melting-point metal, or (5) a combination of an organic compound containing an alkaline-earth metal and a low-melting-point metal-based compound. This conductive paste preferably contains selenium (Se) or tellurium (Te) as the low-melting-point metal and a Se compound or a Te compound as the low-melting-point metal-based compound.